Date: January 7, 2010

Title: Are You Sure Those Are Moons by Jove?


Podcaster: Mark Thompson

Organization: Galileo 1610

Description: If it’s a clear night tonight, January 7, 2010, go outside just after sunset and look to the southwest. The brightest object in the night sky will be the planet Jupiter–shining at a magnitude of -2, only 570 million miles away from us on earth. Let’s take a trip on our imaginary time machine, exactly 400 years ago to the day, or night, and find out what Galileo Galilei was doing.

Bio: Mark Thompson, a professional cantor and amateur astronomer, has appeared as Galileo on radio, at community theatres and libraries, public schools, colleges and universities throughout the country. He has performed as Galileo for civic organizations, astronomy association conventions, marketing and outreach programs as well as private events and parties since 1996.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. Please consider sponsoring an episode or two, or three.


If it’s a clear night tonight, January 7, 2010, go outside just after sunset and look to the southwest. Hold out your arm as far as you can. Now make a fist. Extend your thumb down so that it appears to touch the horizon. Then stretch your pinky finger up as much as you can. Now depending on your geographical location, not to mention the size of your hand, at this point your pinky should pretty much be pointing very close to the brightest object in the night sky, for the moon has not yet risen. And that glowing object would be the planet, Jupiter–shining at a magnitude of -2, only 570 million miles away from us on earth.

Now, if you have a telescope, maybe you’re even fortunate enough to have one of those wonderful little Galileoscopes that were made to commemorate the International Year of Astronomy last year—put that scope on a steady tripod, place a low-power eyepiece in your focuser, so that it magnifies Jupiter by roughly 25 times, and there you should see four small, but fairly bright discs in a relatively straight line, two on each side of the planet. Those would be the Jovian satellites, Jupiter’s moons: descending from west to east: Ganymeade, Europa on the west, then Io and Callisto on the east.

Now let’s take a trip on our imaginary time machine, exactly 400 years ago to the day, or night. We are in Padua, in the Venetian Republic of Italy, and we are the guest of a middle-aged professor with a reddish-brown beard, who has the most melodic name we’ve ever heard: Galileo Galilei. We stand with him on his veranda which overlooks his garden. Over the wall we can see in the distance the domes of the San Antonio Basilica. And this fellow is so infused with energy, that the chill of the wintry night air seems to dissipate with every word he speaks as he gives us a tour of the star-studded Milky Way with his newly-fashioned perspicillum, for indeed it has not yet been given the name “telescope”. We’ve just visited the brilliant Seven Sisters or Pleiades, moved to Orion’s belt, and now Galileo is pointing his spyglass for the first time at the planet Jupiter, now two handbreadths above the eastern horizon. And this is what he sees: Let’s listen to his own words, well- they’ve been translated into English from Latin as they first appeared in the soon-to-be-world famous pamphlet “Siderius Nuncius-The Starry Messenger”

“On the seventh day of January of the present year, 1610, at the first hour of the following night, when I was viewing the constellations of the heavens through a spyglass, the planet Jupiter presented itself to my view. As I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, owing to want of power in my other spyglass. That is, three little stars, small but very bright, were near the planet. Although I believed them to belong to the number of fixed stars, yet they made me wonder somewhat, because they seemed to be arranged exactly in a straight line parallel to the ecliptic, and to be brighter than the rest of the stars equal to them in magnitude. Their position with reference to one another and to Jupiter was as follows: On the east side there were two stars, and a single one towards the west. The star that was furthest towards the east, and the western star, appeared rather larger than the third. I scarcely troubled at all about the distance between them and Jupiter, for, as I have already said, at first I believed them to be fixed stars.”

Now if we were somehow able to bring our modern-day telescope with its much superior resolution to Padua that night of January 7, 1610, we could have shown Professor Galilei that in fact those two stars that he had identified to the east of Jupiter were in fact, three bodies. The innermost ones, which we know as Io and Europa were merely 5 arc seconds apart and thus they appeared as one since Galileo could not see their separation with his crude spyglass. At this point in time, by the way, Galileo had not yet intuited that these heavenly bodies were satellites of Jupiter, remember he thought them to be fixed stars.

But let’s listen to his forthcoming description:

“However, turning to the same investigation on January 8th, led by what, I do not know—I found a very different arrangement. The three starlets were now all to the west of Jupiter, closer together, and at equal intervals from one another.”

Now with our wide-field modern day eyepiece we could have demonstrated again to the Professor that in fact there was an additional starlet far to the east of Jupiter, and he would have been amazed and grateful to be corrected. Well, maybe it’s a stretch to say that he would have been grateful. You see, Galileo quite enjoyed being the chosen one. But, to make a very long story short, GG continued his observations of Jupiter on subsequent nights and it would take him another week to find the fourth moon of Jupiter, but by then he had already concluded that those were moons by Jove, well, to be more accurate, moons going around Jove. Again, Signore Galilei:

And thus, on the eleventh of January, I had now decided beyond all question that there existed in the heavens three stars wandering about Jupiter as do Venus and Mercury about the sun, and this became plainer than daylight from observations on similar occasions which followed. (Watch out Galileo, your Copernicanism is showing!) Nor were there three such stars; four wanderers complete their revolutions about Jupiter, and of their alterations as observed more precisely later on we shall give a description here.

Indeed he goes on to describe and graphically depict in meticulous detail these motions through the beginning of March and with a final flourish, he reminds us: “Such are the observations concerning the four Medicean planets recently first discovered by… me.”

And this is the legacy which Galileo has bequeathed to us—nothing would give him more pleasure than to know, 400 years later, that we continue to be fascinated by the dance of these planets and have even sent probes named after him, to photograph and investigate the surface of Jupiter’s moons.

So you think you can dance? Don’t forget to share this Jovian spectacle with others.

Good day and Clear Skies–

End of podcast:

365 Days of Astronomy
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